Good Girl Gets a Motor (was: Correcting my EARLIER Instrument Notes on EV Power)


 

What kind of motor and battery did you get, Josh? Figure that you can only use half the rated capacity of the battery without damaging it.

One confusing bit is the Peukert effect, which means your battery charge goes further when you draw juice from it more slowly, and vice versa. Just grabbing imaginary numbers out of the air... If the battery goes down to 50% in an hour when you draw 20 amps from it, when you draw only 10 amps from it you don't double the time to get down to 50%, you actually get _more than_ two hours run time. So it pays to go slower. Even if you got the "best bang for the buck" cheap Minn Kota 30 lb. with resistors to control the motor speed (Ohm's law).

Get a good "smart charger" if you want to get the best life out of your battery.

See you on the lake.

On 4/7/2021 10:22 AM, Josh wrote:
Just got Good Girl a small trolling motor and battery so I've been going through trying to learn about all this, it's easy to get confused.
_._,_._,_
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John <jkohnen@boat-links.com>
Better to trust the man who is frequently in error than the one who is never in doubt. (Eric Sevareid)
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Josh
 

It's a Minn Kota 30lb thrust and the battery is a Duracell deep cycle 80a/h. I saw what I think you were describing with the Peukert effect when I was looking at runtime and how drastically it reduced as the draw increased. Reminds me of the energy to go from displacement to planning. 
Think the charger I have is a smart charger but will be checking.


On Wed, Apr 7, 2021, 3:50 PM John Kohnen <jkohnen@...> wrote:
What kind of motor and battery did you get, Josh? Figure that you can
only use half the rated capacity of the battery without damaging it.

One confusing bit is the Peukert effect, which means your battery charge
goes further when you draw juice from it more slowly, and vice versa.
Just grabbing imaginary numbers out of the air... If the battery goes
down to 50% in an hour when you draw 20 amps from it, when you draw only
10 amps from it you don't double the time to get down to 50%, you
actually get _more than_ two hours run time. So it pays to go slower.
Even if you got the "best bang for the buck" cheap Minn Kota 30 lb. with
resistors to control the motor  speed (Ohm's law).

Get a good "smart charger" if you want to get the best life out of your
battery.

See you on the lake.

On 4/7/2021 10:22 AM, Josh wrote:
> Just got Good Girl a small trolling motor and battery so I've been going
> through trying to learn about all this, it's easy to get confused.
> _._,_._,_

--
John <jkohnen@...>
Better to trust the man who is frequently in error than the one who is
never in doubt. (Eric Sevareid)


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That's a good setup. You'd have to spend a lot more money to get a trolling motor with electronic speed control, and 30 lb. thrust should move Good Girl just fine. The runtime graphic is necessarily just a rough guide, but it does take into account Mr. Peukert. That motor should draw about 30 amps at full "throttle". It'd be nice to do some tests with an ammeter to see what it draws under typical conditions pushing Good Girl at the different speed settings. I did such a test with a 30 lb. Minn Kota on Pickle years ago but, alas, the results got lost in a computer upgrade since then. One of these days I'm gonna rig up a portable ammeter for doing trials like that, so maybe I can go out in Good Girl sometime for some test runs.

Get a digital voltmeter (cheap from harbor Freight will do) and keep an eye on the battery voltage now and then as you go along. I charged Tuffy's flooded cell batteries if they got down to 12 volts under light load -- my usual pace getting back to the dock after a sail -- which I figured was close enough to 50% discharge. The only really accurate way to to tell the state of charge by voltage is to let the battery rest for several hours before testing. Not very practical for us! <g> Here's some basic info about the Peukert effect and what I think is a practical voltage/state of charge chart, because it agrees with what I've been doing. <g>:

https://groups.io/g/oregoncoots/files/Motors/BatteryBasix.png

Don't sweat it if you have to go a little below 12 volts to get back to the dock, but recharge the battery right away, and promise to try to treat the battery better in the future. <g>

"Smart" battery chargers are getting cheaper and more common. I wouldn't be surprised to discover that they're the most common kind above the cheapest ones nowadays.

On 4/8/2021 10:32 AM, Josh wrote:
It's a Minn Kota 30lb thrust and the battery is a Duracell deep cycle 80a/h. I saw what I think you were describing with the Peukert effect when I was looking at runtime and how drastically it reduced as the draw increased. Reminds me of the energy to go from displacement to planning.
Think the charger I have is a smart charger but will be checking.
--
John <jkohnen@boat-links.com>
If you can't be a good example, then you'll just have to be a horrible warning. (Catherine the Great)
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Josh
 

Batteries Plus Bulbs tried to get me to go with a battery about 1/2 as many a/h but I wanted enough to have some play room beyond my expected use.
What you said about charging promptly is something I also saw on Minn Kota's website. I'll have to check out the voltmeters you mentioned and I noticed C.G. regs say we have to have a fuse in line.
Be glad to let you take Good Girl out for trials on the power usage like you mentioned. 


 

The CG reg only applies to permanent installations. You don't need to worry about that with a clamp-on trolling motor. The CG doesn't mess with little boats like ours for things like that anyway, and you'll never see them on Fern Ridge. <g> By "voltmeter" I meant a cheap Harbor Freight digital multimeter, like this one:

https://www.harborfreight.com/7-function-digital-multimeter-63759.html

That group 24 battery you got is about as small as I'd want to go. There may come a time when you have to motor quite a ways, especially if the wind dies and you have to get back to the ramp by sundown. <harrumph>

On 4/9/2021 10:22 AM, Josh wrote:
Batteries Plus Bulbs tried to get me to go with a battery about 1/2 as many a/h but I wanted enough to have some play room beyond my expected use.
What you said about charging promptly is something I also saw on Minn Kota's website. I'll have to check out the voltmeters you mentioned and I noticed C.G. regs say we have to have a fuse in line.
Be glad to let you take Good Girl out for trials on the power usage like you mentioned.
--
John <jkohnen@boat-links.com>
People who have tried it, tell me that a clear conscience makes you very happy and contented; but a full stomach does the business quite as well, and is cheaper, and more easily obtained. (Jerome K. Jerome)
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Electri-Cal
 

A good size 27 is about right on a sail boat, with the next step up as two batteries of six volts each, one on each side.  Longer run time with twin 6 volts, for when the winds don't blow, or you want to motorsail back in.  The last lead acid batts. I got lately, have extra power, longer run time, at better cost to power, like 160 minutes at 25 amps - in 80 deg. temps.  They do that for $130.  My Minn Kotas use 15 amps each, smaller ones use less.  

Rough figures look to be about 100 amps before stop, so a lengthwise trip on Paulina lake with previous batteries.  Launched from the lodge, to camp, then back again to dock is about what I got on one trip with a lighter sail boat.  I would think most coot sail boats would be fine with that range.  Weather was calm on that trip, so evening winds mean you might need extra power to make it home, upwind still a problem.

I got stuck downwind on Dorena a few years back, had the Minn Kota at half throttle to help the sails get back to the dock.  Damn near didn't make that, keep power reserves for safety.  La Verne swamped the kayak and swam it back to shore, I could see her in the lake center, but no power to get out and back.  This at sundown !!  Enough to scare the poop out of us both !!

Later,  Cal 


Josh
 

Any thoughts/ tips on adding a solar panel to the system?